The IUPAC name of palbociclib is 6-acetyl-8-cyclopentyl-5-methyl-2-{[5-(1-piperazinyl)-2-pyridinyl]amino}pyrido[2,3-d]pyrimidin-7(8H)-one.
Palbociclib is represented by the following chemical structure according to Formula (I):

Palbociclib (also known as PD-0332991 or 6-acetyl-8-cyclopentyl-5-methyl-2-(5-piperazin-1-yl-pyridin-2ylamino)-8H-pyrido[2,3-d]pyrimidin-7-one) is a selective inhibitor of the cyclin-dependent kinases CDK4 and CDK6. The active pharmaceutical ingredient is administered in the treatment of ER-positive and HER2-negative breast cancer. It is reported to be used in combination with a further drug (letrozole) for patients with estrogen receptor-positive advanced breast cancer.
The active pharmaceutical ingredient palbociclib is known from WO 03/062236 A1.
In WO 2014/128588 A1 two solid forms of palbociclib are discussed, which are denominated as Form A and Form B. According to said application Form A can be used in pharmaceutical formulations, but only if it is prepared in a particle size distribution above specified thresholds, because of the strong electrostatic charging of smaller particles.
With regard to the Form B a X-ray powder diffractogram and 13C CPMAS spectrum was disclosed. However, no further information is given, neither to its synthesis nor to its properties. Thus, the alleged Form B cannot be regarded as being enabled by the above-mentioned application.
Palbociclib (Form A) is reported to be a yellow to orange powder with pKa of 7.4 (the secondary piperazine nitrogen) and 3.9 (the pyridine nitrogen). Palbociclib in form of the free base is allegedly practically insoluble in water. However, when subjected to acidic conditions, such as at or below pH 4, palbociclib allegedly shows good solubility.
One option to enhance the solubility is the formation of a palbociclib base or acid addition salt. Known salts like hydrochloride, dihydrochloride, and 2-hydroxyethyl-sulfonate (WO 2005/005426 A1) show good water solubility. However, they are reported to have certain disadvantages. For example, the hydrochloride of palbociclib features low crystallinity, and the dihydrochloride of palbociclib is hygroscopic. Further the 2-hydroxyethylsulfonate of palbociclib raises potential safety problems (because the permissible daily exposure limit for the anion will likely be exceeded in therapeutic applications).
Consequently, there is still a need for forms of palbociclib in form of solid state having superior properties. Hence, it was an object of the present invention to overcome the drawbacks of the above-mentioned prior art.